The search for new and effective antineoplastic and antihyperlipidemic agents is continually expanding into new areas of technology. Recent advancements have been made in the field of boron chemistry and more specifically, amine borane compounds such as Me.sub.3 N.BH.sub.2 COOH and R.sub.1 R.sub.2 NH.BH.sub.2 C(O)NHR.sub.3 have been shown to exhibit antitumor and antihyperlipidemic activities. See U.S. Pat. No. 4,587,359. However, these compounds contain a relatively large amount of boron which is potentially toxic and therefore hazardous to the health of an animal or human undergoing treatment. These compounds are also poor carriers and have problems with solubility. A compound which contains a relatively small amount of boron but still exhibits significant antitumor and antihyperlipidemic activity has yet to be developed.
It is well known in traditional non-boron organic chemistry to synthesize a peptide bond by the reaction of various amino acids in the presence of a coupling reagent. One of the most widely used coupling reagents in organic peptide synthesis is N,N'-dicyclohexylcarbodiimide (DDC). [H. D. Law, "The Organic Chemistry of Peptides", Wiley-Interscience, New York, N.Y., p. 90 (1970). H. D. Jakubke and H. Jeschkeit, "Akadamie-Verlag", Berlin, p. 108 (1977)]. In the organic coupling reaction, the nitrogen atom of one amino acid attacks the carbonyl carbon of the carboxylic acid portion of the other amino acid in order to form a peptide bond.
Peptide bond formation with a boron-containing amino acid such as Me.sub.3 N.BH.sub.2 CO.sub.2 H would not be expected because of the formal negative charge on the boron atom next to the carbonyl group. The replacement of carbon in an amino acid by boron has a dramatic effect on the relative K.sub.a for the boron amino acids versus the normal amino acids since boron is much less electronegative than carbon. The pK.sub.a for Me.sub.3 N.BH.sub.2 CO.sub.2 H is about six log units more basic than the carboxylic acid group in Me.sub.3 N.sup.+ CH.sub.2 CO.sub.2 H. [K. H. Scheller, R. B. Martin, B. F. Spielvogel and A. T. McPhail, "Inorganic Chemica Acta" 57, 227 (1982)]. The resulting formal negative charge on the boron atom renders the carbonyl carbon in Me.sub.3 N.BH.sub.2 CO.sub.2 H much less susceptible to nucleophilic attack by an incoming amine group of an amino acid. Therefore, it would not be expected that a boron dipeptide analog could be prepared utilizing traditional organic coupling reagents and amino acid techniques. A process for producing a boron dipeptide analog has heretofore not been developed.